NG Firewall's Web Filter has an option for Enforcing Safe Search on Popular Search Engines (Apps > Web Filter > Advanced), which blocks search terms in search engines. More information on how this works and how to configure it can be viewed here: https://support.untangle.com/hc/en-us/articles/216916747-Why-do-I-still-see-inappropriate-content-with-SafeSearch-enabled-
However, NGFW does not determine what is marked as inappropriate or offensive, as we use categories and searches the search engines themselves mark as inappropriate. This can cause issues where some searches still reveal results that are not appropriate to the viewer. In order to block these specific search terms, you can block by term (Apps > Web Filter > Search Terms). This will block specified terms when searching via search engines.
NOTE: In order to block by Search Term, SSL Inspector must be installed and configured as explained in the article linked above.